Simple polysaccharides, such as bacterial levan (BL), not conjugated to protein elicit a thymus independent (TI) response. Although the TI response is clonally restricted, the repertoire of antibodies that can be produced is much greater as evidenced by the appearance of new clonotypes produced in mice immunized with polysaccharide-protein complexes. The focus of this investigation is on the mechanisms or events that determine which of the potential antibodies is actually expressed. Our earlier studies examining the isoelectric focusing (IEF) pattern of IgG antibodies produced in response to immunization with BL in genetically defined mouse strains demonstrated that the complexity of the IgG response is regulated by at least one C57BL/6 gene, designated spectrotype regulation gene 1 (Sr-1), and is not linked to the Igh gene complex, MHC, or coat color. Studies in progress are focused on mapping the Sr-1 gene to a murine chromosome. A large group of BALB/c X CB6F1 backcross mice has been bred, immunized with BL, and typed as Sr-1 positive or negative by IEF analysis of anti-inulin antibodies in the serum. Spleen DNA lysates have been prepared from all the backcross mice. A set of genetic markers in the mouse genome has been selected for the initial screening of the chromosomes and the mice are being genotyped using simple sequence repeat (SSR)-PCR analysis. Testing of all the selected markers with DNA from the parental mouse strains is in progress. SSR-PCR analysis of backcross 1 (BC1 mice has been completed for chromosomes 1, 2, 3, 4, and 5. Thus far, no linkage between Sr-1 and one of the selected markers has been detected. In addition, a group of backcross 2 mice has been bred for further analysis. Mapping and characterization of a gene (Sr-1) which regulates the expression of antibody diversity following immunization with a polysaccharide antigen will contribute to a better understanding of the immune response to polysaccharide vaccines in man and will facilitate the production of more effective vaccines.